1. Field of the Invention
This invention relates to a method for identifying defective heat fusion joints connecting two pipe segments from outside of the fitting. This invention further relates to the use of acoustic signals for identifying defective heat fusion joints. In one aspect, this invention relates to an acoustic method for measuring the ovality of a pipe segment connected with a heat fusion joint fitting. In one aspect, this invention relates to an acoustic method for measuring the angle of insertion of a pipe segment in a heat fusion joint fitting.
2. Description of Related Art
Plastic pipe segments are typically joined together by fusion joints. Such joints are made by heating the outside of a plastic pipe end and the inside of a plastic pipe fitting, e.g. coupling or tee, pushing the heated plastic pipe end into the heated fitting and allowing the resulting joint to cool. Electrofusion couplings are preassembled and then heated with wires embedded in the electrofusion coupling. A proper fusion joint is one where the pipe and the fitting are aligned along a common axis, i.e. coaxial alignment, and the pipe segment is substantially round. However, sometimes the pipe is inserted into the fitting at an angle and sometimes the pipe has an unacceptable amount of ovality which precludes the desired quality of joining around the periphery of the plastic pipe. To date, it has not been possible to “see” through the fitting and pipe to determine if either of these undesirable conditions exist after the joint is complete.
Ovality is defined as the difference between the largest diameter (semi-major axis) and smallest diameter (semi-minor axis) of a pipe divided by the average diameter times 100%. Testing has shown that poor quality joints result if the pipe ovality exceeds about 5%. Currently, there is a standard in place that requires ovality to be no more than 5% and pipes having an ovality which exceeds this amount are not to be used in a fusion joint. There is no standard specifying the maximum angle at which a pipe may be inserted into a fitting and still be acceptable. However, it is known that too large an angle can result in failure of the joint.
Conventionally, the ovality of a plastic pipe is measured by calipers on the outside of the pipe before the joint is formed. If the measured ovality exceeds 5%, then the joint should not be made. However, it is not now possible to determine the ovality of the pipe by mechanical means after the joint has been made. The angle the pipe makes with the fitting can be measured mechanically after the joint is formed. However, the same tightly confined spaces that led to the excessive angle also make it difficult to make this measurement.